The (meth)acrylic acids are a compound easy to polymerize, and the polymerization is promoted in particular under high temperature conditions. If the (meth)acrylic acids are polymerized in the production process thereof, the apparatus/piping equipment are clogged by a solid matter produced by the polymerization and in an extreme case, the operation cannot be continued. In addition, even if shutdown due to clogging of the apparatus/piping equipment is avoided, there arise many problems, for example, an increase in the cleaning frequency of the apparatus/piping equipment during normal operation or in the cleaning load at the time of periodic maintenance.
To cope with these problems, when distilling (meth)acrylic acids, a polymerization inhibitor is added so as to prevent the polymerization thereof. However, many polymerization inhibitors have a low vapor pressure compared with (meth)acrylic acids, and the polymerization inhibitor added is often scarcely contained in the gas of volatilized (meth)acrylic acids. Even though the polymerization inhibitor is not contained, as the density of (meth)acrylic acids in a gas state is low, it is expected that a polymerization reaction does not substantially occur. However, when the gas is once condensed and turns into a condensate, the condensate has high polymerizability and brings about clogging, etc. of the apparatus. The apparatus in which the state above is most likely produced is a condenser for cooling and condensing a distillate gas from a heating distillation apparatus. Accordingly, various studies are being made so as to prevent (meth)acrylic acids from polymerization in the condenser.
For example, Patent Document 1 describes a method for distilling and purifying 2-hydroxy (meth)acrylate, wherein a vapor of 2-hydroxy (meth)acrylate is turned into a condensate in a direct contact-type condenser, part of the obtained condensate is further cooled, the cooled condensate is fed as a spray solution into the direct contact-type condenser so as to swiftly lower the temperature of the vapor of 2-hydroxy (meth)acrylate, and a polymerization inhibitor is added to the spray solution, thereby suppressing generation of a polymerization product in the condenser.
In Patent Document 2, for distilling and purifying an easily polymerizable compound, when a vapor of an easily polymerizable compound is cooled in a condenser and turned into a condensate, with use of a vertical multi-tube heat exchanger as the condenser, part of the obtained condensate is circulated to the gas introduction side of the vertical multi-tube heat exchanger and uniformly sprayed on the upper tube plate to wet the inner surface of a condenser tube (heat transfer tube) with the condensate flowing down inside the tube, thereby preventing a distillate gas in an overheated state from coming into direct contact with the condenser tube to produce a polymerization product. In Patent Document 2, it is stated that a polymerization inhibitor may be added to the condensate circulated.
In this connection, as stated in Non-Patent Document 1, in a vertical multi-tube heat exchanger, for construction reasons, it is a common practice to slightly protrude a heat transfer tube end from a tube plate surface at the time of fixing the heat transfer tube to the tube plate. Accordingly, in the vertical multi-tube heat exchanger described in Patent Document 2, it is thought that upper ends of all heat transfer tubes protrude from the upper tube plate surface.
On the other hand, Patent Document 3 demonstrates that compared with a case where a multi-tube heat exchanger having a heat transfer tube protruded by 1 mm on average from a tube plate surface is used as the vertical multi-tube heat exchanger for cooling and condensing a gas of an easily polymerizable compound, use of a multi-tube heat exchanger having a heat transfer tube which is not protruded from a tube plate surface makes it possible to suppress accumulation of a polymerization product inside the multi-tube heat exchanger. In Patent Document 3, it is stated that the heat transfer tube is not protruded from the upper tube plate and a process fluid can thereby be prevented from staying on the upper tube plate surface, as a result, polymerization of an easily polymerizable compound can be suppressed.
In Patent Document 4, as a multi-tube heat exchanger with ensuring sufficient strength at the junction between the heat transfer tube and the upper tube plate, which suppresses production of a polymerization product on an upper tube plate surface as well as inside a heat transfer tube, thereby getting rid of damage to the heat exchanger, and which enables continuous operation for a long period of time, a multi-tube heat exchanger wherein out of a plurality of heat transfer tubes, some are a heat transfer tube having an upper end which is not protruded from the upper tube plate and others are a heat transfer tube having an upper end protruded from the upper tube plate is described.